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Signal Timing Manual - Second Edition (2015)

Chapter: Chapter 10 - Preferential Treatment

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Suggested Citation:"Chapter 10 - Preferential Treatment ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 10 - Preferential Treatment ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 10 - Preferential Treatment ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 10 - Preferential Treatment ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Chapter 10. Preferenal Treatment CHAPTER 10 PREFERENTIAL TREATMENT CONTENTS 10.1 TYPES OF PREFERENTIAL TREATMENT ................................................................... 10-1 10.2 INTRODUCTION TO PREFERENTIAL TREATMENT ............................................... 10-2 10.2.1 Detection Requirements .................................................................................................... 10-3 10.2.2 Signal Timing Strategies .................................................................................................... 10-5 10.2.3 Strategic Recovery ............................................................................................................... 10-9 10.2.4 Data Logging ........................................................................................................................ 10-10 10.3 PREFERENTIAL TREATMENT ADVANCEMENTS ................................................. 10-10 10.4 PREEMPTION AND PRIORITY .................................................................................... 10-12 10.4.1 Preemption Settings ......................................................................................................... 10-12 10.4.2 Priority Settings ................................................................................................................. 10-14 10.5 PREEMPTION CONSIDERATIONS FOR RAIL ......................................................... 10-14 10.5.1 Entry into Railroad Preemption .................................................................................. 10-16 10.5.2 Advance Preemption Considerations ........................................................................ 10-18 10.5.3 Scheduling Other Calls for Service ............................................................................. 10-20 10.5.4 Railroad Preemption Dwell (or Hold)....................................................................... 10-22 10.5.5 Railroad Preemption Controller Settings ................................................................ 10-23 10.6 PREFERENTIAL TREATMENT CONSIDERATIONS FOR EMERGENCY VEHICLES ...................................................................................................................... .. 10-23 10.7 PREFERENTIAL TREATMENT CONSIDERATIONS FOR TRANSIT................... 10-23 10.8 PREFERENTIAL TREATMENT CONSIDERATIONS FOR TRUCKS .................... 10-25 10.9 REFERENCES .................................................................................................................... 10-27 Signal Timing Manual, Second Edion

Chapter 10. Preferenal Treatment LIST OF EXHIBITS Exhibit 10-1 Preferential Treatment Applications ................................................................. 10-1 Exhibit 10-2 Preferential Treatment Process ........................................................................... 10-2 Exhibit 10-3 Preferential Treatment Detection Equipment ............................................... 10-3 Exhibit 10-4 Types of Preferential Treatment Detection .................................................... 10-4 Exhibit 10-5 Green/Phase Extension ........................................................................................... 10-6 Exhibit 10-6 Red Truncation/Early Green ................................................................................. 10-6 Exhibit 10-7 Phase Insertion ........................................................................................................... 10-7 Exhibit 10-8 Sequence Change ....................................................................................................... 10-8 Exhibit 10-9 Phase Skipping ............................................................................................................ 10-8 Exhibit 10-10 Example of Post-Preferential-Treatment Recovery .................................... 10-9 Exhibit 10-11 Accommodating Multiple Preferential Treatment Requests through Scheduling ................................................................................................ 10-11 Exhibit 10-12 Limited Service (Dwell) Phases ........................................................................ 10-12 Exhibit 10-13 Recovery (Exit) Phases ......................................................................................... 10-13 Exhibit 10-14 Railroad Preemption at an At-Grade Crossing ............................................ 10-15 Exhibit 10-15 Simultaneous Preemption with Maximum RTT..........................................10-17 Exhibit 10-16 Simultaneous Preemption with No RTT......................................................... 10-17 Exhibit 10-17 Advance Preemption Time .................................................................................. 10-18 Exhibit 10-18 Example of Preempt Trap When RTT Is Zero and There Is No Mitigation ............................................................................................................. 10-19 Exhibit 10-19 Two Preempt Operation (Advance and Simultaneous) .......................... 10-19 Exhibit 10-20 Schedule-Based Two Preempt Operation (Recommended Practice) ...................................................................................................................... 10-20 Exhibit 10-21 Time to Green “Scheduled” Preemption for LRT ....................................... 10-21 Exhibit 10-22 Multiple Transit Vehicles on Approach ......................................................... 10-24 Exhibit 10-23 Example of Overlapping Transit Priority Routes ...................................... 10-25 Exhibit 10-24 Example TSP Decision Tree ................................................................................ 10-25 Exhibit 10-25 Truck Position in Queue ....................................................................................... 10-26 Exhibit 10-26 Truck Signal Priority Detection ........................................................................ 10-26 Exhibit 10-27 Green Extension for Truck Signal Priority ................................................... 10-27 Signal Timing Manual, Second Edion

Chapter 10. Preferenal Treatment LIST OF EXHIBITS Exhibit 10-1 Preferential Treatment Applications ................................................................. 10-1 Exhibit 10-2 Preferential Treatment Process ........................................................................... 10-2 Exhibit 10-3 Preferential Treatment Detection Equipment ............................................... 10-3 Exhibit 10-4 Types of Preferential Treatment Detection .................................................... 10-4 Exhibit 10-5 Green/Phase Extension ........................................................................................... 10-6 Exhibit 10-6 Red Truncation/Early Green ................................................................................. 10-6 Exhibit 10-7 Phase Insertion ........................................................................................................... 10-7 Exhibit 10-8 Sequence Change ....................................................................................................... 10-8 Exhibit 10-9 Phase Skipping ............................................................................................................ 10-8 Exhibit 10-10 Example of Post-Preferential-Treatment Recovery .................................... 10-9 Exhibit 10-11 Accommodating Multiple Preferential Treatment Requests through Scheduling ................................................................................................ 10-11 Exhibit 10-12 Limited Service (Dwell) Phases ........................................................................ 10-12 Exhibit 10-13 Recovery (Exit) Phases ......................................................................................... 10-13 Exhibit 10-14 Railroad Preemption at an At-Grade Crossing ............................................ 10-15 Exhibit 10-15 Simultaneous Preemption with Maximum RTT..........................................10-17 Exhibit 10-16 Simultaneous Preemption with No RTT......................................................... 10-17 Exhibit 10-17 Advance Preemption Time .................................................................................. 10-18 Exhibit 10-18 Example of Preempt Trap When RTT Is Zero and There Is No Mitigation ............................................................................................................. 10-19 Exhibit 10-19 Two Preempt Operation (Advance and Simultaneous) .......................... 10-19 Exhibit 10-20 Schedule-Based Two Preempt Operation (Recommended Practice) ...................................................................................................................... 10-20 Exhibit 10-21 Time to Green “Scheduled” Preemption for LRT ....................................... 10-21 Exhibit 10-22 Multiple Transit Vehicles on Approach ......................................................... 10-24 Exhibit 10-23 Example of Overlapping Transit Priority Routes ...................................... 10-25 Exhibit 10-24 Example TSP Decision Tree ................................................................................ 10-25 Exhibit 10-25 Truck Position in Queue ....................................................................................... 10-26 Exhibit 10-26 Truck Signal Priority Detection ........................................................................ 10-26 Exhibit 10-27 Green Extension for Truck Signal Priority ................................................... 10-27 Signal Timing Manual, Second Edion Chapter 10. Preferenal Treatment 10-1 CHAPTER 10. PREFERENTIAL TREATMENT The concepts presented throughout this manual have emphasized an outcome based approach to traf ic signal timing. Preferential treatment is an application that can be used at signalized intersections to adjust operations in favor of a particular user. This chapter provides an overview of the signal timing treatments that can be applied to prioritize rail, emergency vehicles, transit vehicles, and trucks. 10.1 TYPES OF PREFERENTIAL TREATMENT The concept of preferential treatment began with the development of traf ic signal preemption, initially designed as a way to immediately alter traf ic signal operations near at-grade railroad crossings. Once vehicle detection improved, preemption applications eventually expanded to include emergency vehicles. Under a preemption sequence, normal operations are immediately interrupted in order to serve the preferred vehicle, without regard for the state of the signal. This can cause disruption of coordination, pedestrian service, and phasing patterns, but the severity of impacts depends on several factors, including the timing parameters, intersection spacing, transition algorithm, level of saturation, duration of preemption, and amount of slack time available in the intersection cycle (1). A limited number of bus transit operations initially used preemption, but this application was largely abandoned due to the frequent disruption of signal timing and negative impacts on coordination and traf ic progression. The desire for preferential treatment of transit eventually led to the development of traffic signal priority concepts. A priority sequence will maintain signal coordination while allowing timing adjustments to accommodate preferred vehicles. A current limitation of priority systems is that they are largely irst-come- irst-serve. Most controllers are constrained by the information they receive about future requests. While some systems can schedule requests using setback detection and communication, requests can also be managed from a leet or transit management center. Exhibit 10-1 provides general de initions for preemption and priority applications. Preferenal Treatment Definion Traffic Signal Preempon Per NTCIP 1202:2005 (2), preempon is the transfer of the normal control (operaon) of traffic signals to a special signal control mode for the purpose of serving railroad crossings, emergency vehicle passage, mass transit vehicle passage, and other special tasks, the control of which requires terminang normal traffic control to provide the service needs of the special task. Traffic Signal Priority Per NTCIP 1211 (3), priority is the preferenal treatment of one vehicle class (such as transit vehicles, emergency service vehicles, or commercial fleet vehicles) over another vehicle class at a signalized intersecon without causing the traffic signal controller to drop from coordinated operaons. Traffic signal preempon interrupts normal operaons (breaking coordinaon) to serve preferred vehicles, while traffic signal priority maintains system coordinaon when serving priority requests. Exhibit 10-1 Preferenal Treatment Applicaons Signal Timing Manual, Second Edion

10-2 Chapter 10. Preferenal Treatment 10.2 INTRODUCTION TO PREFERENTIAL TREATMENT While there are various preferential treatment applications, the procedure for prioritizing a vehicle generally remains the same. There are ive main steps (illustrated in Exhibit 10-2) that deine the preferential treatment process: 1. Upstream Detection. The preferred vehicle sends the system a “request” for preferential treatment through an upstream detector. Depending on the type of controller irmware used in the deployment, this can be a request for immediate service (common in older controller irmware) or for the time of service desired (preferred if available). Types of preferential treatment detection are explained in Section 10.2.1. 2. Transition Selection. Depending on the status of the signal controller (as well as the status of higher priority requests), the controller or central system selects which signal timing transition to apply (e.g., green extension, red truncation). Types of transitions are described in Section 10.2.2. 3. Timing Transition. Preferential treatment is activated, and the controller begins a right-of-way transfer procedure. During this process, the signal controller safely transitions (or terminates if necessary) all phases in conlict with the signal indications requested by the preferred vehicle and transfers the right-of-way to the desired phase(s). 4. Dwell Stage. After the desired signal state is reached, the controller dwells in that state until either the preferred vehicle clears the intersection and is Exhibit 10-2 Preferenal Treatment Process Signal Timing Manual, Second Edi on

10-2 Chapter 10. Preferenal Treatment 10.2 INTRODUCTION TO PREFERENTIAL TREATMENT While there are various preferential treatment applications, the procedure for prioritizing a vehicle generally remains the same. There are ive main steps (illustrated in Exhibit 10-2) that deine the preferential treatment process: 1. Upstream Detection. The preferred vehicle sends the system a “request” for preferential treatment through an upstream detector. Depending on the type of controller irmware used in the deployment, this can be a request for immediate service (common in older controller irmware) or for the time of service desired (preferred if available). Types of preferential treatment detection are explained in Section 10.2.1. 2. Transition Selection. Depending on the status of the signal controller (as well as the status of higher priority requests), the controller or central system selects which signal timing transition to apply (e.g., green extension, red truncation). Types of transitions are described in Section 10.2.2. 3. Timing Transition. Preferential treatment is activated, and the controller begins a right-of-way transfer procedure. During this process, the signal controller safely transitions (or terminates if necessary) all phases in conlict with the signal indications requested by the preferred vehicle and transfers the right-of-way to the desired phase(s). 4. Dwell Stage. After the desired signal state is reached, the controller dwells in that state until either the preferred vehicle clears the intersection and is Exhibit 10-2 Preferenal Treatment Process Signal Timing Manual, Second Edi on Chapter 10. Preferenal Treatment 10-3 detected at a downstream detector or the preferential treatment interval times out (as programmed in the controller). Generally, limited service (in which the controller serves phases not in conlict with the preferred movement) is the preferred mode of dwell because it minimizes intersection delay. 5. Recovery. After the preferred vehicle leaves the intersection, the signal can begin the recovery stage. Recovery (exit) phases are selected by the practitioner and are usually composed of those phases that have been adversely affected by preemption operations. Once the recovery phases have been served, the signal controller reverts to normal operations through irmware-speciic predetermined logic. If the controller operates in coordination, the local timer may require anywhere from one to ive cycles to transition back to master clock coordination, as the preferential treatment strategy inluences the time discrepancy after service. Recovery strategies are described in Section 10.2.3. 10.2.1 Detecon Requirements At a basic level, preferential treatment detection is similar to standard vehicular detection in that it requires (1) the preferred vehicle to be detected in the roadway and (2) the detection to be registered by the trafic control system. An important difference is that each preferred vehicle must be equipped with a request generator that communicates to the trafic control system. Exhibit 10-3 illustrates a preferential treatment detection system, where there is a request generator on the preferred vehicle sending a detection signal to the request server in the signal cabinet (3). Detection technologies differ in how preferential treatment requests are detected, and while some detection technologies register a “check-in,” others emit a continuous priority request call (similar to vehicular presence detection). Exhibit 10-3 Preferenal Treatment Detecon Equipment Signal Timing Manual, Second Edion

10-4 Chapter 10. Preferenal Treatment The detection should be recognized far enough in advance so that the signal controller can conclude other competing activities before serving the request. However, detection too far in advance is not desirable due to the variability in intersection arrival times. Older irmware may only have the capability to serve preferential treatment requests immediately, while more advanced systems can use “time of service desired” (TSD) and “time of estimated departure” (TED) to better manage timing transitions and competing requests. TSD is the estimated arrival time of the preferred vehicle at the stop bar, while TED is the estimated departure time of the preferred vehicle clearing the intersection (3). Note that current detection inputs to the controller use a simple contact closure and not an exchange of data for TSD and TED, which means that TSD and TED requests are sent immediately. Some of the most common types of detection used for preferential treatment are described in Exhibit 10-4 (4). GPS-based detection systems use the most advanced detection technology and are increasingly being implemented. However, infrared detection systems remain the most widely used. Detecon Type Equipment Required Advantages Limitaons Vehicle- Based GPS □ In-vehicle computer that uses GPS to update vehicle locaon connuously □ Field unit in cabinet □ No unobstructed line-of- sight requirement □ Noficaon when a vehicle has cleared the intersecon □ Potenally larger detecon range (i.e., requests received sooner) □ Some systems may not adequately sample vehicle locaons for accurate informaon at closely spaced intersecons □ Acquiring satellites in urban environments can be challenging Hard-Wired Loop □ Transponder aached to underside of vehicle (coded with unique idenficaon numbers for automac vehicle idenficaon) □ Loop of wire embedded in pavement □ Similar to commonly used loop detectors □ No unobstructed line-of- sight requirement □ Relavely easy to implement a downstream check-out detector □ Overall very reliable □ Requires in-pavement detectors, which need to be appropriately placed and maintained □ Distance-based, rather than me-based, requiring an esmate of speed (which may change with traffic condions) Infrared (Light-Based) □ Infrared strobe emier on the vehicle (which can contain a unique idenficaon number for tracking purposes) □ Infrared detectors at each intersecon □ Detecon interface device in the cabinet □ Widely used, allowing regions to ulize uniform systems for emergency and transit vehicles □ Technology has been well tested during its many years in use □ Requires line of sight between the vehicle and detector; effecve operaon can be hindered by roadway geometry, weather problems, and obstrucons such as tree foliage Radio-Based □ Radio frequency (RF) transponders mounted on the vehicle □ Upstream RF tag readers □ RS-232 connector to connect tag reader to signal controller □ No unobstructed line-of- sight requirement □ Requires suitable curbside locaons for tag readers, including mounng locaons, power, and communicaons connecons Exhibit 10-4 Types of Preferenal Treatment Detecon Signal Timing Manual, Second Edion

10-4 Chapter 10. Preferenal Treatment The detection should be recognized far enough in advance so that the signal controller can conclude other competing activities before serving the request. However, detection too far in advance is not desirable due to the variability in intersection arrival times. Older irmware may only have the capability to serve preferential treatment requests immediately, while more advanced systems can use “time of service desired” (TSD) and “time of estimated departure” (TED) to better manage timing transitions and competing requests. TSD is the estimated arrival time of the preferred vehicle at the stop bar, while TED is the estimated departure time of the preferred vehicle clearing the intersection (3). Note that current detection inputs to the controller use a simple contact closure and not an exchange of data for TSD and TED, which means that TSD and TED requests are sent immediately. Some of the most common types of detection used for preferential treatment are described in Exhibit 10-4 (4). GPS-based detection systems use the most advanced detection technology and are increasingly being implemented. However, infrared detection systems remain the most widely used. Detecon Type Equipment Required Advantages Limitaons Vehicle- Based GPS □ In-vehicle computer that uses GPS to update vehicle locaon connuously □ Field unit in cabinet □ No unobstructed line-of- sight requirement □ Noficaon when a vehicle has cleared the intersecon □ Potenally larger detecon range (i.e., requests received sooner) □ Some systems may not adequately sample vehicle locaons for accurate informaon at closely spaced intersecons □ Acquiring satellites in urban environments can be challenging Hard-Wired Loop □ Transponder aached to underside of vehicle (coded with unique idenficaon numbers for automac vehicle idenficaon) □ Loop of wire embedded in pavement □ Similar to commonly used loop detectors □ No unobstructed line-of- sight requirement □ Relavely easy to implement a downstream check-out detector □ Overall very reliable □ Requires in-pavement detectors, which need to be appropriately placed and maintained □ Distance-based, rather than me-based, requiring an esmate of speed (which may change with traffic condions) Infrared (Light-Based) □ Infrared strobe emier on the vehicle (which can contain a unique idenficaon number for tracking purposes) □ Infrared detectors at each intersecon □ Detecon interface device in the cabinet □ Widely used, allowing regions to ulize uniform systems for emergency and transit vehicles □ Technology has been well tested during its many years in use □ Requires line of sight between the vehicle and detector; effecve operaon can be hindered by roadway geometry, weather problems, and obstrucons such as tree foliage Radio-Based □ Radio frequency (RF) transponders mounted on the vehicle □ Upstream RF tag readers □ RS-232 connector to connect tag reader to signal controller □ No unobstructed line-of- sight requirement □ Requires suitable curbside locaons for tag readers, including mounng locaons, power, and communicaons connecons Exhibit 10-4 Types of Preferenal Treatment Detecon Signal Timing Manual, Second Edion Chapter 10. Preferenal Treatment 10-5 Detecon Type Equipment Required Advantages Limitaons Sound-Based (Siren-Based) □ Siren on vehicle □ Sound detector at intersecon □ No new vehicle equipment □ Less precise control of acvaon Push Buon □ Push buon (e.g., in fire staon) □ Allows acvaon before vehicle departs □ Requires manual acvaon Track Circuits □ Designed by railroad or transit agency □ Advance, simultaneous, and gate-down noficaons provide maximum flexibility of operaon □ Upgrades from simultaneous-only can be very expensive 10.2.2 Signal Timing Strategies How requests for service are accommodated at a particular intersection depends on many variables, including (but not limited to): • Controller irmware capabilities, • Agency policy, • Cycle length, • Complexity of phases, • Trafic on intersecting streets, • Protection of minimum clearance times for pedestrians, • Minimum phase times, and • Accuracy of check-out mechanisms. Regardless of the trafic signal controller’s strategy for providing preferential treatment, it is important to have an understanding of the parameters and expected outcomes. This section explains the various signal timing methods that can be used to accommodate preferential treatment. Some strategies are used with priority and can maintain coordination (e.g., green extension and phase skipping), while others are used with preemption and are more disruptive, typically requiring recovery strategies to return to coordinated operations. It should be noted that if preemption is frequent, transitions back to coordination may result in frequent disruptions, and uncoordinated operations should be considered. 10.2.2.1 Green/Phase Extension Green (or phase) extension is a common strategy used to serve preferential treatment requests. Green extension involves the extension (or holding) of the preferred phase green interval past its normal termination point (as illustrated in Exhibit 10-5). Depending on the type of system and detection scheme, the extension period can be for a ixed duration or until the preferred vehicle has cleared the intersection (subject to a maximum extension). This strategy is designed to prevent long delays for preferred vehicles that are anticipated to arrive near the end of the green interval. Signal Timing Manual, Second Edion

10-6 Chapter 10. Preferenal Treatment 10.2.2.2 Red Truncaon/Early Green Red truncation (or early green) is a preferential treatment strategy that shortens the duration of non-preferred phases in order to return earlier than normal to the green interval of the preferred phase (as illustrated in Exhibit 10-6). Exhibit 10-5 Green/Phase Extension Exhibit 10-6 Red Truncaon/Early Green Signal Timing Manual, Second Edion

10-6 Chapter 10. Preferenal Treatment 10.2.2.2 Red Truncaon/Early Green Red truncation (or early green) is a preferential treatment strategy that shortens the duration of non-preferred phases in order to return earlier than normal to the green interval of the preferred phase (as illustrated in Exhibit 10-6). Exhibit 10-5 Green/Phase Extension Exhibit 10-6 Red Truncaon/Early Green Signal Timing Manual, Second Edion Chapter 10. Preferenal Treatment 10-7 In this strategy, the duration of some or all of the non-preferred phases can be reduced. How quickly the trafic signal controller can return to the preferred phase is constrained by the minimum green time and clearance requirements of the non- preferred vehicular and pedestrian phases. This strategy is less beneicial than green extension and may be undesirable at locations with competing preferred vehicles. 10.2.2.3 Phase Inseron Phase insertion involves the activation of a special, dedicated phase that is not served during normal (non-preferred) operations and is only displayed when a preferred vehicle has been detected at the intersection. This strategy is commonly used to provide service to lanes that are dedicated to preferred vehicles only (e.g., an exclusive left-turn lane into a transit transfer point). This strategy is also used to support queue jumps that allow preferred vehicles to enter a downstream link ahead of the normal trafic stream. Exhibit 10-7 shows the use of phase insertion with a queue jump strategy. Note that the dedicated queue jump phase is Phase 9. Depending on whether the controller can accommodate more than eight phases, this phase insertion strategy may need to be managed differently (e.g., unused phase, if available) than shown in Exhibit 10-7. 10.2.2.4 Sequence Change With sequence change, the order of the signal phases is altered to provide more immediate service to the preferred vehicle (as shown in Exhibit 10-8). In this example, changing a left-turn phase from leading to lagging reduces the wait time of the preferred vehicle. However, the practitioner should ensure that a sequence change will not cause operational issues; the phase sequence considerations from normal (non-preferred) operations should not be ignored (e.g., if a lead-lag sequence was chosen to accommodate intersection geometry). See Chapter 5 for guidance on phase sequence. Exhibit 10-7 Phase Inseron Signal Timing Manual, Second Edion

10-8 Chapter 10. Preferenal Treatment 10.2.2.5 Phase Skipping Phase skipping (shown in Exhibit 10-9) forgoes service to (or skips) non-preferred phases that would normally be served, in order to serve the preferred phase more quickly. For example, phase skipping may skip the protected interval at a protected- permitted left-turn movement when needed. Because of the potential impact this strategy can have on delays to non-preferred movements, the practitioner should consider tradeoffs carefully when implementing this strategy. Exhibit 10-8 Sequence Change Exhibit 10-9 Phase Skipping Signal Timing Manual, Second Edion

10-8 Chapter 10. Preferenal Treatment 10.2.2.5 Phase Skipping Phase skipping (shown in Exhibit 10-9) forgoes service to (or skips) non-preferred phases that would normally be served, in order to serve the preferred phase more quickly. For example, phase skipping may skip the protected interval at a protected- permitted left-turn movement when needed. Because of the potential impact this strategy can have on delays to non-preferred movements, the practitioner should consider tradeoffs carefully when implementing this strategy. Exhibit 10-8 Sequence Change Exhibit 10-9 Phase Skipping Signal Timing Manual, Second Edion Chapter 10. Preferenal Treatment 10-9 10.2.3 Strategic Recovery To maintain operations at an intersection, a practitioner should consider a post- preferential-treatment “recovery” (or “exit”) strategy. While preferential treatment can help meet speciic objectives by prioritizing certain users, there are tradeoffs for the overall intersection operations. A clear understanding of the desired outcomes and tradeoffs of preferential treatment is necessary when selecting a recovery strategy. For example, Exhibit 10-10 illustrates an intersection under typical emergency vehicle preemption. If the minimization of intersection delay and queuing is the primary operational objective, then the most appropriate recovery strategy could be to serve the minor street approach before returning to normal operations. A recovery strategy can help mitigate the effects of preferential treatment for non- preferred users, particularly when • Preferential treatment has a long duration, leading to long delays for movements that are prohibited during preferential treatment. • Preferential treatment requests are frequent (i.e., back-to-back or near back- to-back), leading to substantially shortened or skipped phases or movements, such that long, disproportionate queues and delays occur. There are a variety of strategies that can be applied at an intersection after preferential treatment. These recovery options are increasingly becoming available within trafic signal controller irmware. Those that can dynamically respond to prevailing conditions are the most advanced, but a dynamic recovery strategy is not necessary for all locations or time periods. Recovery strategies include • Return to Normal Operations. Return to normal trafic signal timing; this is often the default. • Return to Free Operations. Return to free operations for a deined period of time; this allows the signal to leverage longer green intervals and clear vehicular queues. Exhibit 10-10 Example of Post-Preferenal- Treatment Recovery Signal Timing Manual, Second Edion

10-10 Chapter 10. Preferenal Treatment • Return to Coordinated Operations. Tracks coordinated timing parameters in the background for immediate return to coordination, without the need for a transition period to correct offsets. • Return to Alternate Plan. Return to coordinated operations through the use of alternate timing parameters (e.g., cycle, splits, offsets, and/or phase order) for a deined period of time, in order to clear queues. • Return to Interrupted Phase(s) (Priority Return). Return to vehicular and/or pedestrian phases that were interrupted at the onset of the preferential treatment request. • Return to Deined Phase(s). Invariable return to deined vehicular and/or pedestrian phase(s) upon exit of preferential treatment. • Queue Delay Recovery. Dynamic return to movements that have the highest delay, volume, or combination of both (based on detection); most effective where frequent priority calls occur and if coordination does not necessitate immediate restoration. 10.2.4 Data Logging Much like any signal timing, monitoring and maintenance of preferential treatment will help the system continue to operate as desired. Controller logs and centralized recording software are available to help practitioners log and better understand preferential treatment operations. Data logs can provide information about when, where, how, and who requested preferential treatment so that preferential treatment rights and signal timing settings may be actively managed. 10.3 PREFERENTIAL TREATMENT ADVANCEMENTS The historical approach to preferential treatment has been to serve only one request at a time. However, trafic signal controller irmware is increasingly featuring advanced preferential treatment options that allow practitioners to assign levels of priority to users (1). For example, a bus can be assigned a higher level of priority than a truck. This approach overcomes many of the limitations of traditional preferential treatment by scheduling calls based on the assigned priorities. With a schedule-based approach, the controller irmware assigns priorities to requests as they are received. The highest priority request is always served at its scheduled time. If possible, the irmware then continues to serve other requests in order of priority. Lower priority requests that interfere with higher priority requests must yield to the higher priority users. While schedule-based preferential treatment does not eliminate conlicting requests, the treatment facilitates the resolution of conlicts based on priorities. In many cases, an immediate response to a call is unnecessary, and eficiencies may be realized with the application of preferential treatment scheduling (5, 6). Other beneits may include (5) • Provision of adequate pedestrian clearance times, • Selection of transition modes that minimize trafic disruptions, and • Accommodation of large vehicles (e.g., trucks) on high-speed or downhill approaches. Logs and central tracking soware are important to the ac ve management of an effec ve preferen al treatment system. Scheduling and serving mul ple preferen al requests is an emerging field of prac ce. There are limited prac cal applica ons due to limita ons in most controller firmware. Signal Timing Manual, Second Edion

10-10 Chapter 10. Preferenal Treatment • Return to Coordinated Operations. Tracks coordinated timing parameters in the background for immediate return to coordination, without the need for a transition period to correct offsets. • Return to Alternate Plan. Return to coordinated operations through the use of alternate timing parameters (e.g., cycle, splits, offsets, and/or phase order) for a deined period of time, in order to clear queues. • Return to Interrupted Phase(s) (Priority Return). Return to vehicular and/or pedestrian phases that were interrupted at the onset of the preferential treatment request. • Return to Deined Phase(s). Invariable return to deined vehicular and/or pedestrian phase(s) upon exit of preferential treatment. • Queue Delay Recovery. Dynamic return to movements that have the highest delay, volume, or combination of both (based on detection); most effective where frequent priority calls occur and if coordination does not necessitate immediate restoration. 10.2.4 Data Logging Much like any signal timing, monitoring and maintenance of preferential treatment will help the system continue to operate as desired. Controller logs and centralized recording software are available to help practitioners log and better understand preferential treatment operations. Data logs can provide information about when, where, how, and who requested preferential treatment so that preferential treatment rights and signal timing settings may be actively managed. 10.3 PREFERENTIAL TREATMENT ADVANCEMENTS The historical approach to preferential treatment has been to serve only one request at a time. However, trafic signal controller irmware is increasingly featuring advanced preferential treatment options that allow practitioners to assign levels of priority to users (1). For example, a bus can be assigned a higher level of priority than a truck. This approach overcomes many of the limitations of traditional preferential treatment by scheduling calls based on the assigned priorities. With a schedule-based approach, the controller irmware assigns priorities to requests as they are received. The highest priority request is always served at its scheduled time. If possible, the irmware then continues to serve other requests in order of priority. Lower priority requests that interfere with higher priority requests must yield to the higher priority users. While schedule-based preferential treatment does not eliminate conlicting requests, the treatment facilitates the resolution of conlicts based on priorities. In many cases, an immediate response to a call is unnecessary, and eficiencies may be realized with the application of preferential treatment scheduling (5, 6). Other beneits may include (5) • Provision of adequate pedestrian clearance times, • Selection of transition modes that minimize trafic disruptions, and • Accommodation of large vehicles (e.g., trucks) on high-speed or downhill approaches. Logs and central tracking soware are important to the ac ve management of an effec ve preferen al treatment system. Scheduling and serving mul ple preferen al requests is an emerging field of prac ce. There are limited prac cal applica ons due to limita ons in most controller firmware. Signal Timing Manual, Second Edion Chapter 10. Preferenal Treatment 10-11 Several researchers have evaluated the impact of scheduling requests on normal signal operations (6, 7, 8, 9, 10). To illustrate the schedule-based concept, Exhibit 10-11 shows how preferential treatment can accommodate a call for truck priority prior to emergency vehicle preemption. It is important to recognize that preemption is not scheduled (as it will always take precedence at an intersection), but the schedule-based approach provides more ef icient transitions prior to preemption, so that other preferential treatment requests can be served. In this example, the service requests (from the truck and emergency vehicle) are received at different times and will require service at different times. Using schedule- based preferential treatment, the controller is able to continue serving the current phase in order to accommodate the truck, skip the next phase, and then accommodate the emergency vehicle. In order for the controller  irmware to decide if both requests Exhibit 10-11 Accommodang Mulple Preferenal Treatment Requests through Scheduling Signal Timing Manual, Second Edion

10-12 Chapter 10. Preferenal Treatment can be served (or if one request needs to take precedent), a TSD is required for each priority vehicle prior to its arrival at the intersection. 10.4 PREEMPTION AND PRIORITY Every trafic signal controller implements preemption and priority in a slightly different manner. Hence, it is critical that practitioners become familiar with their irmware and shop test all applications before implementation. Section 10.4 provides information about preemption and priority controller settings that apply to all vehicle types. Speciic preemption and priority controller settings that apply only to rail, emergency vehicles, transit vehicles, or trucks are discussed throughout the remainder of the chapter. 10.4.1 Preemp on Sengs Preemption settings that must be selected and incorporated into the controller include but are not limited to the following (11): • Preemption Phase(s). The phase (or phases) to be served as part of preemption. • Limited Service (Dwell) Phases. All phases with movements that do not conlict with the preemption movement should be permitted to continue to operate while the signal dwells (as illustrated in Exhibit 10-12) in order to minimize delay and queuing. Limited service phases need to be identiied for each preemption movement. Note that the ability to cycle through limited service phases depends on the length of the preemption event (i.e., more limited service phases will be served during longer preemption events, such as long freight trains). • Recovery (Exit) Phases. Recovery phases are activated after termination of the dwell period. The phases that require rapid recovery after preemption should be designated as recovery phases (as illustrated in Exhibit 10-13). Exhibit 10-12 Limited Service (Dwell) Phases Signal Timing Manual, Second Edi on

10-12 Chapter 10. Preferenal Treatment can be served (or if one request needs to take precedent), a TSD is required for each priority vehicle prior to its arrival at the intersection. 10.4 PREEMPTION AND PRIORITY Every trafic signal controller implements preemption and priority in a slightly different manner. Hence, it is critical that practitioners become familiar with their irmware and shop test all applications before implementation. Section 10.4 provides information about preemption and priority controller settings that apply to all vehicle types. Speciic preemption and priority controller settings that apply only to rail, emergency vehicles, transit vehicles, or trucks are discussed throughout the remainder of the chapter. 10.4.1 Preemp on Sengs Preemption settings that must be selected and incorporated into the controller include but are not limited to the following (11): • Preemption Phase(s). The phase (or phases) to be served as part of preemption. • Limited Service (Dwell) Phases. All phases with movements that do not conlict with the preemption movement should be permitted to continue to operate while the signal dwells (as illustrated in Exhibit 10-12) in order to minimize delay and queuing. Limited service phases need to be identiied for each preemption movement. Note that the ability to cycle through limited service phases depends on the length of the preemption event (i.e., more limited service phases will be served during longer preemption events, such as long freight trains). • Recovery (Exit) Phases. Recovery phases are activated after termination of the dwell period. The phases that require rapid recovery after preemption should be designated as recovery phases (as illustrated in Exhibit 10-13). Exhibit 10-12 Limited Service (Dwell) Phases Signal Timing Manual, Second Edi on Chapter 10. Preferenal Treatment 10-13 • Preemption Number. The preemption number is a unique identiier in the preemption sequence used to assign speciic phases to each preemption request (e.g., rail versus emergency vehicle). Typically, six or more different preemption plans (e.g., two railroad and four emergency vehicle plans) can be implemented. • Preemption Priority. The preemption priority is a numerical value that distinguishes the service priorities of conlicting or overlapping preemption calls on multiple approaches or for multiple users. For example, railroad preemption needs to be programmed for a higher priority than emergency vehicle preemption. Note that if more than one call of equivalent priority is received at a controller, irst-come-irst-serve service applies. • Preemption Duration. Preemption duration is a set value of time for service of a preemption request. Durations may be speciied by a minimum preemption duration, a maximum preemption duration, or the option to hold the preemption while the input is received (i.e., request is active for preemption). Typically, the preemption vehicle is able to check out earlier than the preemption duration. • Preemption Minimum Green and Walk. Minimum green and walk can be set to alternate values during preemption. The minimum green during preemption should not be set less than 2 seconds. • Preemption Flashing Don’t Walk (FDW). In general, the FDW interval should be equal to the duration set under normal operations. However, the Manual on Uniform Trafic Control Devices (MUTCD) permits truncated FDW intervals under speciic conditions (12). • Preemption Delay. The preemption delay parameter delays the start of the preemption interval by a deined value. It is typically set to zero seconds. However, delay may be needed to prevent false preempts at locations where the detection is highly variable. Exhibit 10-13 Recovery (Exit) Phases Signal Timing Manual, Second Edion

10-14 Chapter 10. Preferenal Treatment • Preemption Memory. Preemption memory saves the preemption request until the movement has been served. The signal should normally be operated with preemption memory active to ensure that the call is served. However, this can cause false preempts (or phantom preempts) when the detection is defective. 10.4.2 Priority Sengs When trafic signal priority is being implemented, a practitioner should consider the following priority settings (note that these may be different by mode and for low- versus high-priority requests): • Priority Phasing Sequence. Practitioners should look at modiied phasing sequences closely to ensure undesirable operations are not introduced with priority, such as a yellow trap or indications that violate user expectancy. • Minimum Phase Duration. Practitioners should identify those phases that cannot be truncated during priority. Among the phases for which truncation is permitted, practitioners should specify the minimum duration for which a phase must be served before termination. • Duration of Green/Phase Extension. When timing transitions are applied, practitioners should balance advantages to priority vehicles with the disadvantages to non-priority modes and movements. • Minimum Green Times. Timing adjustments to non-priority phases should not shorten green times beyond reasonable minimum green times. • Pedestrian Intervals. Timing adjustments to non-priority phases should not shorten pedestrian clearance (12). 10.5 PREEMPTION CONSIDERATIONS FOR RAIL When a signalized intersection is located near an at-grade railroad crossing, a practitioner should consider using preemption (in conjunction with railroad warning devices) to clear any vehicular queues that may extend over the tracks (13), as illustrated in Exhibit 10-14. In accordance with the MUTCD, a trafic signal warrants preemption when the at-grade railroad crossing is equipped with active warning devices and is located within 200 feet of an intersection (12). When the at-grade crossing is farther than 200 feet from a signalized intersection, a detailed queuing analysis is recommended to determine whether signal preemption is necessary (14). Additional requirements are contained in the Code of Federal Regulations (CFR) Title 49, Section 234.225, various state laws, and the American Railway Engineering and Maintenance-of-Way Association’s (AREMA’s) Communications & Signals Manual (15). More information about rail preemption can also be found in the Guide for Trafic Signal Preemption Near Railroad Grade Crossing (13). In order for a signalized intersection to use preemption effectively to clear vehicular queues, the signal controller needs to acquire information from the railroad. Modern railroad systems use track circuits to estimate the speed and direction of a train as it enters a speciied detection zone. This allows the system to predict the train’s time of arrival at the at-grade crossing, which is used to calculate when the railroad warning devices (i.e., lights and gates) need to activate and when the trafic signal needs to start preemption. “Simultaneous preemption” is the most common (and most basic) type of preemption, where the track circuit activates the railroad warning devices and requests The primary purpose of rail preempon is to clear any vehicles that are stopped over the tracks before the arrival of a train. In most cases, rail operaons are kept separate from roadway operaons. Signal Timing Manual, Second Edi­on

10-14 Chapter 10. Preferenal Treatment • Preemption Memory. Preemption memory saves the preemption request until the movement has been served. The signal should normally be operated with preemption memory active to ensure that the call is served. However, this can cause false preempts (or phantom preempts) when the detection is defective. 10.4.2 Priority Sengs When trafic signal priority is being implemented, a practitioner should consider the following priority settings (note that these may be different by mode and for low- versus high-priority requests): • Priority Phasing Sequence. Practitioners should look at modiied phasing sequences closely to ensure undesirable operations are not introduced with priority, such as a yellow trap or indications that violate user expectancy. • Minimum Phase Duration. Practitioners should identify those phases that cannot be truncated during priority. Among the phases for which truncation is permitted, practitioners should specify the minimum duration for which a phase must be served before termination. • Duration of Green/Phase Extension. When timing transitions are applied, practitioners should balance advantages to priority vehicles with the disadvantages to non-priority modes and movements. • Minimum Green Times. Timing adjustments to non-priority phases should not shorten green times beyond reasonable minimum green times. • Pedestrian Intervals. Timing adjustments to non-priority phases should not shorten pedestrian clearance (12). 10.5 PREEMPTION CONSIDERATIONS FOR RAIL When a signalized intersection is located near an at-grade railroad crossing, a practitioner should consider using preemption (in conjunction with railroad warning devices) to clear any vehicular queues that may extend over the tracks (13), as illustrated in Exhibit 10-14. In accordance with the MUTCD, a trafic signal warrants preemption when the at-grade railroad crossing is equipped with active warning devices and is located within 200 feet of an intersection (12). When the at-grade crossing is farther than 200 feet from a signalized intersection, a detailed queuing analysis is recommended to determine whether signal preemption is necessary (14). Additional requirements are contained in the Code of Federal Regulations (CFR) Title 49, Section 234.225, various state laws, and the American Railway Engineering and Maintenance-of-Way Association’s (AREMA’s) Communications & Signals Manual (15). More information about rail preemption can also be found in the Guide for Trafic Signal Preemption Near Railroad Grade Crossing (13). In order for a signalized intersection to use preemption effectively to clear vehicular queues, the signal controller needs to acquire information from the railroad. Modern railroad systems use track circuits to estimate the speed and direction of a train as it enters a speciied detection zone. This allows the system to predict the train’s time of arrival at the at-grade crossing, which is used to calculate when the railroad warning devices (i.e., lights and gates) need to activate and when the trafic signal needs to start preemption. “Simultaneous preemption” is the most common (and most basic) type of preemption, where the track circuit activates the railroad warning devices and requests The primary purpose of rail preempon is to clear any vehicles that are stopped over the tracks before the arrival of a train. In most cases, rail operaons are kept separate from roadway operaons. Signal Timing Manual, Second Edi­on Chapter 10. Preferenal Treatment 10-15 trafic signal preemption at the same time. The minimum warning time required for simultaneous preemption is 20 seconds, as discussed below. However, the minimum warning time needed for the railroad warning system to activate the lights (and gates if present) may not be suficient to clear vehicles queued over the tracks. “Advance preemption” is intended to provide additional time for a trafic signal to transition to a phase that will clear the tracks of any vehicles that might be present before the railroad warning devices are activated. In order to know how much time is available for a signal to clear vehicular queues (that may be present over the tracks), a practitioner must irst calculate the minimum warning time (MWT, also known as prescribed warning time) required to activate the railroad warning devices. MWT is deined as the least amount of time that warning devices shall operate prior to the arrival of a train at a railroad at-grade crossing. It is the sum of a minimum time (MT) and a clearance time (CT), as shown in Equation 10-1. According to the AREMA Communications & Signals Manual, the MT is a set value of no less than 20 seconds (15), whereas the CT is variable depending on the minimum track clearance distance. Clearance distance is measured along the highway centerline (or roadway edge if longer) from the near-side railroad warning device to a point that is 6 feet away from the far-side track. There must be 1 second of CT for every 10 feet of Exhibit 10-14 Railroad Preempon at an At- Grade Crossing Signal Timing Manual, Second Edion

10-16 Chapter 10. Preferenal Treatment clearance distance greater than 35 feet. CT can also be added for site-speciic conditions, such as for warning gate delay. MWT = MT (20 seconds) + CT (if required) where MWT = minimum warning time (seconds), MT = minimum time (20 seconds), and CT = clearance time (seconds). In the ield, the observed time that the warning devices are active will often be longer than the design value of MWT because of buffer time (BT) and equipment response time (ERT) added by the railroad. BT is discretionary on the part of the railroad and may be provided in addition to MT and CT. BT is added to accommodate minor variations and ensure that the MWT is always provided. Railroad design times also account for any time required by the railroad equipment to acquire and respond to a train that enters the warning circuit. This additional time in the railroad design calculations is referred to as the ERT and may have several components depending on the complexity of the circuit (or circuits) necessary to operate the system. As noted previously, the most basic form of preemption is called simultaneous preemption, where notiication of an approaching train is forwarded to the trafic signal controller at the same time as the design value of MWT. Modern railroad detection systems can often provide a constant warning time for this notiication. In other words, railroad systems can consistently estimate when trains will arrive at at-grade crossings based on when they are detected at track circuits (as long as train speeds are relatively constant). However, areas with nearby switching are not able to provide consistent warning times (because of the variability in speeds near switching areas). These types of locations require special considerations that are beyond those discussed here and should be designed by those with detailed knowledge of the complexities. 10.5.1 Entry into Railroad Preemp on Entry into preemption is the most critical stage of railroad preemption. In this stage, the right-of-way is transferred to the “track clearance green interval” (TCGI), which is the green interval associated with the signal phase(s) that clear vehicles queued on the tracks. The time required to transfer the right-of-way to the TCGI (from the time preemption is activated in the controller) is known as the right-of-way transfer time (RTT). The RTT plus the time to clear the vehicular queue must be less than the MWT. Otherwise, vehicles could still be on the tracks when the train arrives. When the MWT for simultaneous preempt (generally limited to a maximum of 35 seconds) is inadequate to meet needs, advance preemption becomes necessary (discussed later in this section). Exhibit 10-15 demonstrates the preemption entry concept as part of the simultaneous preemption process. Simultaneous preempt, while common, often requires the undesirable shortening of pedestrian clearance times. As mentioned previously, an extremely important issue is that there is a difference between calculated design values and the actual values provided by the railroad system (and thus timed in the controller). The TCGI may need to be programmed in the controller at a higher value than the calculated design value in order to account for variability. Equa on 10-1 Traffic signal ming for simultaneous preempon should be based on the design value of MWT, not what may be observed in the field. Signal Timing Manual, Second Edi on

10-16 Chapter 10. Preferenal Treatment clearance distance greater than 35 feet. CT can also be added for site-speciic conditions, such as for warning gate delay. MWT = MT (20 seconds) + CT (if required) where MWT = minimum warning time (seconds), MT = minimum time (20 seconds), and CT = clearance time (seconds). In the ield, the observed time that the warning devices are active will often be longer than the design value of MWT because of buffer time (BT) and equipment response time (ERT) added by the railroad. BT is discretionary on the part of the railroad and may be provided in addition to MT and CT. BT is added to accommodate minor variations and ensure that the MWT is always provided. Railroad design times also account for any time required by the railroad equipment to acquire and respond to a train that enters the warning circuit. This additional time in the railroad design calculations is referred to as the ERT and may have several components depending on the complexity of the circuit (or circuits) necessary to operate the system. As noted previously, the most basic form of preemption is called simultaneous preemption, where notiication of an approaching train is forwarded to the trafic signal controller at the same time as the design value of MWT. Modern railroad detection systems can often provide a constant warning time for this notiication. In other words, railroad systems can consistently estimate when trains will arrive at at-grade crossings based on when they are detected at track circuits (as long as train speeds are relatively constant). However, areas with nearby switching are not able to provide consistent warning times (because of the variability in speeds near switching areas). These types of locations require special considerations that are beyond those discussed here and should be designed by those with detailed knowledge of the complexities. 10.5.1 Entry into Railroad Preemp on Entry into preemption is the most critical stage of railroad preemption. In this stage, the right-of-way is transferred to the “track clearance green interval” (TCGI), which is the green interval associated with the signal phase(s) that clear vehicles queued on the tracks. The time required to transfer the right-of-way to the TCGI (from the time preemption is activated in the controller) is known as the right-of-way transfer time (RTT). The RTT plus the time to clear the vehicular queue must be less than the MWT. Otherwise, vehicles could still be on the tracks when the train arrives. When the MWT for simultaneous preempt (generally limited to a maximum of 35 seconds) is inadequate to meet needs, advance preemption becomes necessary (discussed later in this section). Exhibit 10-15 demonstrates the preemption entry concept as part of the simultaneous preemption process. Simultaneous preempt, while common, often requires the undesirable shortening of pedestrian clearance times. As mentioned previously, an extremely important issue is that there is a difference between calculated design values and the actual values provided by the railroad system (and thus timed in the controller). The TCGI may need to be programmed in the controller at a higher value than the calculated design value in order to account for variability. Equa on 10-1 Traffic signal ming for simultaneous preempon should be based on the design value of MWT, not what may be observed in the field. Signal Timing Manual, Second Edi on Chapter 10. Preferenal Treatment 10-17 The components of RTT include the minimum allowable green for the current vehicle green phase, the time required for the pedestrian phase (if agency policy is to serve the pedestrian phase during preemption), and the time required for the yellow change and red clearance intervals of the active phase. If preemption is activated when the controller is in the same phase as the TCGI, the actual RTT is zero (16), as illustrated in Exhibit 10-16. All processes described so far in this section have assumed simultaneous preemption. Simultaneous preemption is suitable when the maximum RTT plus queue clearance time can be accommodated within the designed MWT provided by the railroad. However, there are situations in which simultaneous preemption may not be adequate, such as the following: • When it is undesirable for a pedestrian phase to be truncated. • When longer queue clearance times require an alternative preemption strategy. (Long queue clearance times may result from substantial space between the Exhibit 10-15 Simultaneous Preempon with Maximum RTT Exhibit 10-16 Simultaneous Preempon with No RTT Signal Timing Manual, Second Edion

10-18 Chapter 10. Preferenal Treatment intersection and the crossing or when the queue has vehicles with longer start- up lost times.) In such cases, the practitioner should request additional warning time from the railroad authorities in order to apply advance preemption (illustrated in Exhibit 10-17). The trafic signal operator may have to pay for the cost of the circuit required for the additional warning time (as a longer railroad detection zone will be needed). In the example shown in Exhibit 10-17, RTT happens to equal advance preemption time (which may or may not be the case in all situations). The example design also provides a separation time, which is desirable but not a requirement. Separation time ends the TCGI before the arrival of the train. 10.5.2 Advance Preempon Consideraons While an RTT of zero seconds may give a irst impression of an acceptable scenario, Exhibit 10-18 illustrates that the TCGI can terminate before the lights start to lash and the gates come down (16). In what is referred to as the “preempt trap,” a queue may form over the track before the gates descend. Once the gates lower, the queue will not be served by the controller until preemption is removed, potentially leaving the vehicles stranded on the tracks. In addition, actual advance preemption times may be longer than the design advance preemption times due to decreased speeds as the train approaches the crossing. This extended advance preemption time furthers the negative effects of the preempt trap. In order to eliminate the preempt trap and reduce the resultant safety concerns, several treatments can be implemented using existing railroad and trafic signal technology. One such treatment is the installation of a not-to-exceed timer by the railroad operation that forces the railroad warning devices to activate no later than the end of the design advance preemption time (15). Implementation of the not-to-exceed timer should be coupled with a TCGI duration that is at least equal to the advance preemption time. This strategy ensures that irrespective of both the variability in train speed and the corresponding warning time, the railroad warning devices will be activated before (or with) the TCGI, reducing the potential for vehicle queues over the Exhibit 10-17 Advance Preempon Time Signal Timing Manual, Second Edion

10-18 Chapter 10. Preferenal Treatment intersection and the crossing or when the queue has vehicles with longer start- up lost times.) In such cases, the practitioner should request additional warning time from the railroad authorities in order to apply advance preemption (illustrated in Exhibit 10-17). The trafic signal operator may have to pay for the cost of the circuit required for the additional warning time (as a longer railroad detection zone will be needed). In the example shown in Exhibit 10-17, RTT happens to equal advance preemption time (which may or may not be the case in all situations). The example design also provides a separation time, which is desirable but not a requirement. Separation time ends the TCGI before the arrival of the train. 10.5.2 Advance Preempon Consideraons While an RTT of zero seconds may give a irst impression of an acceptable scenario, Exhibit 10-18 illustrates that the TCGI can terminate before the lights start to lash and the gates come down (16). In what is referred to as the “preempt trap,” a queue may form over the track before the gates descend. Once the gates lower, the queue will not be served by the controller until preemption is removed, potentially leaving the vehicles stranded on the tracks. In addition, actual advance preemption times may be longer than the design advance preemption times due to decreased speeds as the train approaches the crossing. This extended advance preemption time furthers the negative effects of the preempt trap. In order to eliminate the preempt trap and reduce the resultant safety concerns, several treatments can be implemented using existing railroad and trafic signal technology. One such treatment is the installation of a not-to-exceed timer by the railroad operation that forces the railroad warning devices to activate no later than the end of the design advance preemption time (15). Implementation of the not-to-exceed timer should be coupled with a TCGI duration that is at least equal to the advance preemption time. This strategy ensures that irrespective of both the variability in train speed and the corresponding warning time, the railroad warning devices will be activated before (or with) the TCGI, reducing the potential for vehicle queues over the Exhibit 10-17 Advance Preempon Time Signal Timing Manual, Second Edion Chapter 10. Preferenal Treatment 10-19 tracks. In this case, a design modi ication is required for the railroad operations, and the traf ic signal timing must be modi ied. Another approach incorporates a “gate-down preempt” from the railroad signal system into the traf ic signal controller (16, 17). With this treatment, the controller receives the conventional advance preemption call, times any RTT, and holds (dwells in) the TCGI until a second preempt is received from the railroad informing the controller that the warning gates are down. The gate-down preempt (assigned a higher preempt priority) releases the green hold resulting from the advance preempt, and enters a timed TCGI (18). An alternative to the gate-down preempt is the use of simultaneous preempt to time the TCGI. This alternative begins the design TCGI when the warning devices are activated. (Additional time can be added to account for the time required for the gates to descend.) Advance preemption with an RTT of zero and warning-system- active con irmation is illustrated in Exhibit 10-19. Exhibit 10-18 Example of Preempt Trap When RTT Is Zero and There Is No Migaon Exhibit 10-19 Two Preempt Operaon (Advance and Simultaneous) Signal Timing Manual, Second Edion

10-20 Chapter 10. Preferenal Treatment If the controller is serving the TCGI when advance preemption is activated (as shown in the example in Exhibit 10-19), the controller dwells in the TCGI until the warning devices are activated (or gates are down) and then times the designed TCGI. This ensures that the TCGI times after the warning systems are active. This approach requires two preempts from the railroad as well as modi ications to the traf ic signal timing. The traf ic signal controller must be capable of two preempts; the  irst calls and holds the TCGI, and the second implements the timed TCGI. This example is one of several designs that use more than one preempt to create better operation of traf ic signal preemption. 10.5.3 Scheduling Other Calls for Service Traditional preemption (i.e., simultaneous, advance, and advance with warning- system-active or gate-down con irmation) can have negative impacts on other phases, due to the amount of time in the TCGI. If advance preemption is considered a priority request (i.e., a request for service at a certain time scheduled in the future), then a transition into preempt can be more intelligently implemented to achieve more ef icient traf ic signal operations. The ability to do so depends on the functionality of the signal controller, the time required for the RTT, the amount of advance notice required, the reliability of the railroad detection system in providing consistent times, and the time remaining before critical service of the TCGI. Exhibit 10-20 illustrates the schedule- based approach, which relies on knowledge of vehicle (i.e., heavy rail or light rail transit [LRT]) arrival time, current state of signal timing service, and current detection calls. Exhibit 10-20 Schedule-Based Two Preempt Operaon (Recommended Pracce) Signal Timing Manual, Second Edion

10-20 Chapter 10. Preferenal Treatment If the controller is serving the TCGI when advance preemption is activated (as shown in the example in Exhibit 10-19), the controller dwells in the TCGI until the warning devices are activated (or gates are down) and then times the designed TCGI. This ensures that the TCGI times after the warning systems are active. This approach requires two preempts from the railroad as well as modi ications to the traf ic signal timing. The traf ic signal controller must be capable of two preempts; the  irst calls and holds the TCGI, and the second implements the timed TCGI. This example is one of several designs that use more than one preempt to create better operation of traf ic signal preemption. 10.5.3 Scheduling Other Calls for Service Traditional preemption (i.e., simultaneous, advance, and advance with warning- system-active or gate-down con irmation) can have negative impacts on other phases, due to the amount of time in the TCGI. If advance preemption is considered a priority request (i.e., a request for service at a certain time scheduled in the future), then a transition into preempt can be more intelligently implemented to achieve more ef icient traf ic signal operations. The ability to do so depends on the functionality of the signal controller, the time required for the RTT, the amount of advance notice required, the reliability of the railroad detection system in providing consistent times, and the time remaining before critical service of the TCGI. Exhibit 10-20 illustrates the schedule- based approach, which relies on knowledge of vehicle (i.e., heavy rail or light rail transit [LRT]) arrival time, current state of signal timing service, and current detection calls. Exhibit 10-20 Schedule-Based Two Preempt Operaon (Recommended Pracce) Signal Timing Manual, Second Edion Chapter 10. Preferenal Treatment 10-21 The schedule-based approach to planning for the arrival of a train has been used for LRT in Portland, Oregon, to facilitate station-to-station movements. The approach is known as “time to green” (TTG). Exhibit 10-21 (continued on the next page) illustrates this TTG scenario for LRT. Exhibit 10-21 TTG “Scheduled” (connued next page) Preempon for LRT Signal Timing Manual, Second Edion

10-22 Chapter 10. Preferenal Treatment 10.5.4 Railroad Preempon Dwell (or Hold) When the TCGI times out, the controller progresses from the “entry into preemption” stage to the “dwell” (also known as “hold”) stage. During the dwell stage, the signal may operate under one of the following: • Red lash for all phases. • Flashing yellow for major allowable movements and lashing red for allowable minor movements. • Steady red or all-way stop. • Limited service, in which the controller serves the allowable phases. • Rest in green for through movements parallel to the tracks. Signal Timing Manual, Second Edion

10-22 Chapter 10. Preferenal Treatment 10.5.4 Railroad Preempon Dwell (or Hold) When the TCGI times out, the controller progresses from the “entry into preemption” stage to the “dwell” (also known as “hold”) stage. During the dwell stage, the signal may operate under one of the following: • Red lash for all phases. • Flashing yellow for major allowable movements and lashing red for allowable minor movements. • Steady red or all-way stop. • Limited service, in which the controller serves the allowable phases. • Rest in green for through movements parallel to the tracks. Signal Timing Manual, Second Edion Chapter 10. Preferenal Treatment 10-23 As mentioned previously, limited service (in which the controller serves phases not in con lict with the preemption phase) is the most preferred mode of dwell as it minimizes intersection delay. 10.5.5 Railroad Preempon Controller Se ngs For the safe and ef icient service of preemption requests at railroad at-grade crossings, numerous settings in the controller must be programmed. In addition to those explained in Section 10.4.1, the programmable settings necessary for the implementation of railroad preemption include, but are not limited to (11) • TCGI. The TCGI is the green interval associated with the signal phase (or phases) that control movements which may queue over the tracks. A green indication (including a protected left-turn indication) should always be provided for movements associated with the TCGI. • Track Clearance Green Time. Track clearance green time is required for the queue to clear off the tracks before the train arrives. The calculation of track clearance green time is dependent on the distance between the tracks and the intersection stop bar, the composition of the vehicles in the queue, and the relative position of those vehicles with respect to one another. The track clearance green time necessary for the safe operation of traf ic signals during preemption may be calculated using a worksheet developed for the Texas Department of Transportation (19). • Recovery (Exit) Phases. For railroad preemption, the controller commonly serves the phases across the tracks irst in the “recovery” (exit) stage. 10.6 PREFERENTIAL TREATMENT CONSIDERATIONS FOR EMERGENCY VEHICLES Emergency vehicle preferential treatment is used to facilitate the rapid movement of emergency vehicles through traf ic signals. Often, preemption is only used for ire trucks because of the disruption to normal traf ic signal operations that can occur if there are numerous service requests over short periods of time. Other emergency vehicles (e.g., ambulances and police cars) are typically not equipped with preemption capabilities. These emergency vehicles are smaller than ire trucks and more able to maneuver around vehicles in traf ic. More information about emergency vehicle preemption can be found in Trafic Signal Preemption for Emergency Vehicles: A Cross-Cutting Study (20). To minimize the adverse operations effects, practitioners may implement priority routines instead of preemption. 10.7 PREFERENTIAL TREATMENT CONSIDERATIONS FOR TRANSIT Transit signal priority (TSP) is a tool used to improve transit performance and reliability (4). The most common TSP strategies either extend a phase to allow a transit vehicle to pass (i.e., green extension) or terminate con licting phases to allow early service and reduce red time (i.e., red truncation) (6). Green extension facilitates signi icantly less intersection delay than red truncation, and it should be given priority when competing calls exist. However, site-speci ic conditions should also be considered. For example, in order to ensure that desired objectives are met, the location of transit stops may affect how TSP is designed, timed, and implemented. Signal Timing Manual, Second Edion

10-24 Chapter 10. Preferenal Treatment At times, signal systems with TSP may experience competing users, including multiple transit vehicles, pedestrians, bicycles, and other vehicles. Next-generation TSP can intelligently serve competing priority needs (like those shown in Exhibit 10-22) based on lateness, ridership, and route importance, as described below. • Schedule Adherence (i.e., Lateness). Practitioners often desire to provide TSP only to those transit vehicles that are behind schedule. This requires knowledge of transit vehicle locations (i.e., through use of “automatic vehicle location” [AVL] technology) and schedule information (i.e., use of computer-aided dispatch [CAD] system). • Location Information. Practitioners occasionally desire to provide TSP only when a transit vehicle is in service, en route, and not at a transit stop. This requires high-precision GPS and knowledge of transit vehicle locations. Note that the closed position of the door on a transit vehicle is able to serve as a conditional indication for priority requests. • Passenger Counter. Practitioners may desire to serve the route with the highest actual ridership or anticipated ridership, particularly if multiple requests for priority service occur. The ability to track historic and real-time ridership is necessary to enable this dimension of TSP. • Level of Priority. When priority routes overlap, assigning levels of priority across vehicles and routes may be especially important to an operating agency. The system map shown in Exhibit 10-23 provides an example of a transit system for which this distinction may be necessary. Exhibit 10-22Mulple Transit Vehicles on Approach Signal Timing Manual, Second Edion

10-24 Chapter 10. Preferenal Treatment At times, signal systems with TSP may experience competing users, including multiple transit vehicles, pedestrians, bicycles, and other vehicles. Next-generation TSP can intelligently serve competing priority needs (like those shown in Exhibit 10-22) based on lateness, ridership, and route importance, as described below. • Schedule Adherence (i.e., Lateness). Practitioners often desire to provide TSP only to those transit vehicles that are behind schedule. This requires knowledge of transit vehicle locations (i.e., through use of “automatic vehicle location” [AVL] technology) and schedule information (i.e., use of computer-aided dispatch [CAD] system). • Location Information. Practitioners occasionally desire to provide TSP only when a transit vehicle is in service, en route, and not at a transit stop. This requires high-precision GPS and knowledge of transit vehicle locations. Note that the closed position of the door on a transit vehicle is able to serve as a conditional indication for priority requests. • Passenger Counter. Practitioners may desire to serve the route with the highest actual ridership or anticipated ridership, particularly if multiple requests for priority service occur. The ability to track historic and real-time ridership is necessary to enable this dimension of TSP. • Level of Priority. When priority routes overlap, assigning levels of priority across vehicles and routes may be especially important to an operating agency. The system map shown in Exhibit 10-23 provides an example of a transit system for which this distinction may be necessary. Exhibit 10-22Mulple Transit Vehicles on Approach Signal Timing Manual, Second Edion Chapter 10. Preferenal Treatment 10-25 An operating agency may choose to create a decision tree that prioritizes transit vehicles based on several of the characteristics described above. Exhibit 10-24 illustrates a simple example of conditional TSP with integrated CAD/AVL data from the City of Portland, Oregon. 10.8 PREFERENTIAL TREATMENT CONSIDERATIONS FOR TRUCKS If reduced intersection delay is a desired outcome of traf…ic signal timing, then having a truck at the beginning of the queue is an undesirable scenario (as depicted in Exhibit 10-25). The primary objective of truck signal priority is to provide more time for trucks to pass through an intersection in order to reduce the probability that a truck will be positioned as the …irst vehicle at the stop bar. When used effectively, truck signal priority may result in the following bene…its: • Reduction of truck stops and delay. • Potential reduction of truck red-light running. Exhibit 10-23 Example of Overlapping Transit Priority Routes Exhibit 10-24 Example TSP Decision Tree Signal Timing Manual, Second Ediƒon

10-26 Chapter 10. Preferenal Treatment • Safer phase termination for trucks (i.e., decision zone protection). • Increased vehicular capacity of the intersection through reduced truck start-up lost time. • Potential decrease in total intersection delay when truck priority is on a coordinated phase (as a result of the added green time to the major trafic movement). Typically, truck signal priority is implemented on higher-speed approaches through the placement of dual upstream vehicle detectors that respond only to vehicles of a minimum length, traveling at a minimum speed. Truck drivers tend to experience longer decision zones than passenger car drivers (typically up to 8 seconds from the stop bar). For truck signal priority, effective detection is placed beyond, or upstream of, the decision zone (see Exhibit 10-26). Additional information on detector considerations can be found in Design and Installation Guidelines for Advance Warning Systems for End- of-Green Phase at High-Speed Trafic Signals (21). Source: Adapted from Northwest Signal Supply Exhibit 10-25 Truck Posion in Queue Exhibit 10-26 Truck Signal Priority Detecon Signal Timing Manual, Second Edion

10-26 Chapter 10. Preferenal Treatment • Safer phase termination for trucks (i.e., decision zone protection). • Increased vehicular capacity of the intersection through reduced truck start-up lost time. • Potential decrease in total intersection delay when truck priority is on a coordinated phase (as a result of the added green time to the major trafic movement). Typically, truck signal priority is implemented on higher-speed approaches through the placement of dual upstream vehicle detectors that respond only to vehicles of a minimum length, traveling at a minimum speed. Truck drivers tend to experience longer decision zones than passenger car drivers (typically up to 8 seconds from the stop bar). For truck signal priority, effective detection is placed beyond, or upstream of, the decision zone (see Exhibit 10-26). Additional information on detector considerations can be found in Design and Installation Guidelines for Advance Warning Systems for End- of-Green Phase at High-Speed Trafic Signals (21). Source: Adapted from Northwest Signal Supply Exhibit 10-25 Truck Posion in Queue Exhibit 10-26 Truck Signal Priority Detecon Signal Timing Manual, Second Edion Chapter 10. Preferenal Treatment 10-27 Truck decision zones are calculated in the truck priority algorithm, and a priority call for service is placed during that time. If the request can be accommodated, the phase is extended through a low-priority request (as illustrated in Exhibit 10-27). No early green is provided for truck signal priority, and, like TSP, only minor changes are made to signal phasing and timing if coordination is to be retained. 10.9 REFERENCES 1. Nelson, E. J., and D. Bullock. Impact of Emergency Vehicle Preemption on Signalized Corridor Operation: An Evaluation. In Transportation Research Record: Journal of the Transportation Research Board, No. 1727, Transportation Research Board of the National Academies, Washington, D.C., 2000, pp. 1–11. 2. National Transportation Communications for ITS Protocol. Object Deinitions for Actuated Trafic Signal Controller (ASC) Units. NTCIP 1202:2005, v02.19, 2005. 3. National Transportation Communications for ITS Protocol. Object Deinitions for Signal Control and Prioritization. NTCIP 1211, v01, 2008. 4. Smith, H. R., B. Hemily, and M. Ivanovic. Transit Signal Priority (TSP): A Planning and Implementation Handbook. ITS America, Washington, D.C., 2005. 5. Head, L., D. Gettman, and Z. Wei. Decision Model for Priority Control of Traf›ic Signals. In Transportation Research Record: Journal of the Transportation Research Board, No. 1978, Transportation Research Board of the National Academies, Washington, D.C., 2006, pp. 169–177. Exhibit 10-27 Green Extension for Truck Signal Priority Signal Timing Manual, Second Edion

10-28 Chapter 10. Preferenal Treatment 6. He, Q., L. Head, and J. Ding. Heuristic Algorithm for Priority Trafic Signal Control. In Transportation Research Record: Journal of the Transportation Research Board, No. 2259, Transportation Research Board of the National Academies, Washington, D.C., 2011, pp. 1–7. 7. Ma, W., Y. Liu, and X. Yang. A Dynamic Programming Model for Bus Signal Priority with Multiple Requests. Paper 11-2851, Presented at 90th Annual Meeting of the Transportation Research Board, Washington, D.C., 2011. 8. Ghanim, M., F. Dion, and G. Abu-Lebdeh. Integration of Signal Control and Transit Signal Priority Optimization in Coordinated Network Using Genetic Algorithms and Artiicial Neural Networks. Paper 09-3063, Presented at 88th Annual Meeting of the Transportation Research Board, Washington, D.C., 2009. 9. Zhou, G., and A. Gan. Design of Transit Signal Priority at Signalized Intersections with Queue Jumper Lanes. Journal of Public Transportation, Vol. 12, No. 4, 2009, pp. 117–132. 10. Christofa, E., and A. Skabardonis. Trafic Signal Optimization with Transit Signal Priority: Application to an Isolated Intersection. Paper 11-3092, Presented at 90th Annual Meeting of the Transportation Research Board, Washington, D.C., 2011. 11. Bonneson, J., S. Sunkari, and M. Pratt. Trafic Signal Operations Handbook, Second Edition. Report FHWA/TX-11/0-6402-P1, Texas Department of Transportation, Austin, Texas, 2011. 12. Manual on Uniform Trafic Control Devices for Streets and Highways, 2009 Edition. United States Department of Transportation, Federal Highway Administration, Washington, D.C., 2009. 13. Venglar, S. P., M. S. Jacobson, S. R. Sunkari, R. J. Engelbrecht, and T. Urbanik II. Guide for Trafic Signal Preemption Near Railroad Grade Crossing. Report FHWA/TX- 01/1439-9, Texas Department of Transportation, Austin, Texas, 2000. 14. Preemption of Trafic Signals at or Near Railroad Grade Crossings with Active Warning Devices: A Recommended Practice. Trafic Engineering Council Committee, TENC-4M-35. Institute of Transportation Engineers, Washington, D.C., 1997. 15. Communications & Signals Manual of Recommended Practices, 2012 Edition. American Railway Engineering and Maintenance-of-Way Association (AREMA), Lanham, Maryland, 2012. 16. Sun, X., T. Urbanik II, S. Skehan, and M. Ablett. Improved Highway-Railway Interface for the Preempt Trap. In Transportation Research Record: Journal of the Transportation Research Board, No. 2080. Transportation Research Board of the National Academies, Washington, D.C., 2008, pp. 1–7. 17. Yohe, J. R., and T. Urbanik II. Advance Preempt with Gate-Down Conirmation: Solution for Preempt Trap. In Transportation Research Record: Journal of the Transportation Research Board, No. 2035, Transportation Research Board of the National Academies, Washington, D.C., 2007, pp. 40–49. 18. Engelbrecht, R., S. Sunkari, T. Urbanik II, S. Venglar, and K. Balke. The Preempt Trap: How to Make Sure You Do Not Have One. Report 1752-9, Texas Transportation Institute, Arlington, Texas, 2002. Signal Timing Manual, Second Edion

Chapter 10. Preferenal Treatment 10-29 19. Engelbrecht, R. J., K. N. Balke, S. R. Sunkari, and S. P. Venglar. Research Report on Improved Traf ic Signal Operation Near Railroad Grade Crossing with Active Devices. Report FHWA/TX-03/0-4265-1, Texas Department of Transportation, Austin, Texas, 2005. 20. Traf ic Signal Preemption for Emergency Vehicles: A Cross-Cutting Study. Report FHWA-JPO-05-010, Federal Highway Administration, United States Department of Transportation, 2006. 21. Messer, C. J., S. R. Sunkari, and H. A. Charara. Design and Installation Guidelines for Advance Warning Systems for End-of-Green Phase at High-Speed Traf ic Signals. Report FHWA/TX-04/0-4260-2, Texas Department of Transportation, Austin, Texas, 2003. Signal Timing Manual, Second Edion

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 812: Signal Timing Manual - Second Edition, covers fundamentals and advanced concepts related to signal timing. The report addresses ways to develop a signal timing program based on the operating environment, users, user priorities by movement, and local operational objectives.

Advanced concepts covered in the report include the systems engineering process, adaptive signal control, preferential vehicle treatments, and timing strategies for over-saturated conditions, special events, and inclement weather.

An overview PowerPoint presentation accompanies the report.

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